The present invention relates to gas turbine, and more specifically to casings surrounding at least one hot-gas space between a compressor stage and a turbine stage.
In the case of axial-throughflow gas turbines, as a rule, the one or more compressor stages and the one or more turbine stages are arranged on a single shaft. The highly compressed and heated air flowing out of the compressor is supplied to a combustion chamber located within the turbine casing between the compressor stage and turbine stage. Due to the high pressure values and temperatures occurring in this region, the turbine casing is exposed to high load.
The development of high-compression compressors with rising compressor final temperatures leads to increasingly more stringent requirements for the mechanical and thermal stability of the turbine casing. High-grade materials having improved properties must be found and used for the thermal and mechanical loading which increases with a rising pressure ratio. At the same time, ever larger separating-flange screw connections of the turbine casing have to be provided, in order to withstand these loads. Both of these factors increase the cost of the turbine power plants considerably.
Another limiting factor is the manufacturing methods which are employed in the field of industrial gas turbines and in which the outer shells forming the turbine casing are cast. Due to the system employed, however, the mechanical and thermal load-bearing capacity of turbine casings produced by casting methods of this kind is limited.
The present invention provides a turbine casing for an axial-throughflow gas turbine, which turbine casing can be produced cost-effectively and withstands very high pressures and temperatures. Thus, the turbine casing is capable of being operated without difficulty in the region of a compressor final pressure of more than 30 bar at temperatures of 550 to 570xc2x0 C. 
The turbine casing according to the invention surrounds at least one hot-gas space between a compressor stage and a turbine stage. The casing is provided with an outer shell as an external boundary, and an inner component, which is provided separately from the outer shell and which separates the hot-gas space from the outer shell with an intermediate space defined between the outer shell and the inner component. The inner component is connected to the outer shell by two axial interfaces, in such a way that the intermediate space is sealed off relative to the hot-gas space.
The turbine casing according to the invention is thus composed of an outer shell and of an inner component. As a result of the arrangement of the two integral parts, the intermediate space formed between the inner component and the outer shell has a lower pressure and a lower temperature than the hot-gas space surrounded by the inner component. This is made possible, in particular, by the intermediate space being sealed off from the hot-gas space. A predetermined pressure can be set in this intermediate space by suitable feeds to the intermediate space.
The division of the turbine casing according to the invention into an outer shell and an inner component enables the thermal and mechanical loads occurring during operation to be apportioned to the two components. In this case, the inner component, also referred to as the hot-gas component, is designed in such a way that it withstands both the circumferential stresses caused by the pressure difference between the hot-gas space and the intermediate space, and the high temperature prevailing in the hot-gas space. This hot-gas component is therefore manufactured preferably from a high-grade material.
The outer shell must have a sufficiently rigid design to be capable of transmitting the static forces of the gas turbine and of withstanding the pressure difference between the intermediate space and the ambient atmosphere. The temperature which acts on the outer shell is markedly reduced because of the separation from the hot-gas space by the inner component and the intermediate space. This thermal load may be additionally counteracted by suitable cooling-air routed to the intermediate space formed between the inner component and the outer shell. This also reduces the phenomenon of xe2x80x9cbowingxe2x80x9d which can occur in steam and gas turbines and is caused as a rule by deformation of the stator.
A turbine casing constructed in accordance with the invention can be operated at compressor final pressures of more than 30 bar and the associated high temperatures. As a result of the outer shell only having to meet reduced requirements from those of conventional turbine casings, the outer shell can be produced by means of conventional casting methods and simple materials, while high-grade materials are necessary only for the inner component exposed to the high temperature and pressure ranges.
In one advantageous embodiment of the turbine casing according to the invention, the inner component is connected to the outer shell by means of surface pressure acting in the axial direction. The axial direction is a direction parallel to the central axis of the turbine casing. In this case, the outer shell has preferably two inwardly continuous projections or webs that form axial interfaces between which the inner component is placed. The inner component has sufficient flexibility in the axial direction such that it maintains a tight seal against the axial interfaces with the outer shell over the entire operating cycle of the gas turbine. The sealing effect is achieved preferably by metallic surfaces. The axial interfaces of the outer shell and the surfaces of the inner component which come into contact with the axial interfaces are provided with metallic sealing surfaces. The outer shell and webs forming the axial interfaces must have a sufficiently rigid design to absorb the axial forces that result from the surface pressure created during metallic sealing. As a result of this refinement, the turbine casing according to the invention is easily produced.
In a further refinement of the turbine casing, the materials for the outer shell and for the inner component are selected such that, during operation, there is sufficient surface pressure between the interfaces of the components to achieve a good seal. The thermal longitudinal expansion coefficient of the material for the inner component is preferably selected to be lower than that for the outer shell. Different thermal expansions resulting from the different temperatures acting on the two components can be compensated for in this manner. The materials are selected in such a way that the sealing effect between the inner component and the outer shell does not decrease during operation.
A medium can be supplied under pressure into the intermediate space between the inner component and outer shell, for example, a pressure of 16 bar can be maintained in the intermediate space in the case of a pressure of 32 bar in the hot-gas space. In this case, the inner component and the outer shell only have to be capable in each case of withstanding a pressure difference of 16 bar.
The turbine casing according to the invention also makes it possible so that, even under high pressure conditions of the compressor and with large diameters of the components, smaller separating-flange screw connections and simpler materials and geometries can be selected for the outer shell and the inner component. This, too, leads to a reduction in the costs for producing a turbine casing of this type.
A further advantage of the present invention is the simple production of the casing, in which the inner component merely has to be clamped between the two axial interfaces. There is no need, in this case, for further connection techniques which could lead to thermal stresses or cracking.